An experimental investigation was conducted on the flow of air-water mixtures in a number of horizontal rectangular Lucite test sections, whose aspect ratios varied from two to sixteen. These test sections were assembled in various combinations to permit the study of the effect of a sudden change in flow area on the significant flow parameters. A technique was developed for measuring the volume fraction of each phase, making use of the difference of the attenuation characteristics of the liquid and the vapor for gamma radiation. This technique was evaluated making use of a number of Lucite mockups with known void distributions. Information is presented on the factors influencing the volume fractions, flow patterns, and slip ratios in the channels studied. The two-phase pressure drop is compared with existing correlations, and a relationship predicting this pressure drop, in terms of the liquid volume fraction, is derived. The energy losses associated with an abrupt expansion or contraction were studied. A correlation based on the experimental results is presented to permit the prediction of these losses. (auth)